1. Field of the Invention
This invention pertains to acoustic well logging and more particularly to the development of an acoustical logging signal that will enhance a preselected component of an acoustical wave induced into a formation borehole during a well-logging operation.
2. Description of the Prior Art
Acoustical well logging has long been a particularly satisfactory means for developing valuable information about various formation anomalies and interfaces. However, it is also well-known that compressional acoustic well logging is a much easier technique to utilize than well logging utilizing other acoustical waves, because compressional waves are normally larger in amplitude than shear wave components. In addition, they travel faster along a formation path, and therefore arrive earlier at receivers located for detecting reflected and refracted waves than other components (and, hence compressional waves are not so subject to noise or being hidden in reverberations). However, other wave components of acoustical waves do contain information that may not be included in data derived from the use of compressional waves. For example, one of the other wave fronts that yields valuable information is the shear wave front.
A shear wave is developed in a formation as a result of oscillatory reverberations in a medium at right angles to the direction of travel of the wave, which is in contradistinction to a compressional wave. The stiffness of the medium determines how well the medium sustains shear wave propagation, whereas the medium stiffness has less effect on supporting compressional wave propagation. Hence, one popular use of shear wave logging is in the detection of formation fractures, where liquids, gases and spaces occur.
It is also true that a particular rock type of structure produces a characteristic trace as a result of compressional wave logging, and another characteristic trace as a result of shear wave logging. The lithological characteristics of various rock types can be catalogued by these compressional wave and shear wave tracings. Hence, another use of shear wave logging is to determine the lithological properties of formations by comparing traces with catalogued standard tracings.
Since shear wave data is useful and shear waves tend to have a small amplitude in comparison with compressional waves, it is desirable to enhance the shear waves.
It has been discovered that shear waves can be enhanced by being introduced into the wall of a formation borehole at an angle, rather than normal to such wall. In fact, some angles of incidence greatly enhance shear wave amplitude propagation. One suitable structure for introducing acoustic energy at an angle of incidence to a borehole, although not for shear wave enhancement propagation, is shown in U.S. Pat. No. 3,504,759, Cubberly, wherein a conventional acoustic transducer unit 23 projects its acoustic energy at a rotating reflecting surface 28 to cause the waves to be reflected, as surface 28 is rotated over 360 degrees.
U.S. Pat. No. 3,974,476, Cowles, discloses the use of a conventional acoustic transducer unit 21, which directs its acoustic wave longitudinally toward a single reflecting surface 22. The reflected wave pass out of opening 23 and impinge perpendicularly upon the borehole wall.
U.S. Pat. No. 4,184,562, Bakamjian, discloses the use of a centralized transucer 6 for irradiating a conical reflector longitudinally disposed therefrom so that the reflected waves from the reflector enter peripherally into the adjacent borehole over a range of 360 degrees. The disclosed system does not utilize a stacked piezoceramic transducer element or any other means of optimizing the frequency of operation for enhancing shear wave well logging.
Because the preferred embodiment of the present invention uses a frequency enhancing transducer element comprising in part a stacked array of piezoceramic elements, it is noted that U.S. Pat. No. 3,539,980, Massa, does disclose an electroacoustic transducer for producing sound waves including a plurality of stacked piezoceramic elements 24-27, and a vibrational plate 28 for unspecified material located at one end and an inertial mass 29 at the other. The patent does not relate to well logging and does not disclose an optimum frequency for such use or how to produce a desirable frequency by changing the dimensions of the elements, the vibrational plate or the inertial mass.
It is therefore a feature of the present invention to provide an improved apparatus useful in well logging, and particularly with respect to shear wave well logging, including a transducer having stacked piezoceramic rings and a metallic end piece which can be modified to produce a desirable frequency, and a downstream omnidirectional reflector for irradiating the adjacent borehole with selectable collimated, focused or dispersed acoustic energy at a desirable and selectable angle of incidence, all of which enhances the production of shear wave components in a borehole.
It is another feature of the present invention to provide an improved apparatus of similar construction as a receiving transducer.